1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus such as an optical disk apparatus and the like for recording/reproducing information by irradiating a recording medium with light beams. More particularly, the invention relates to an optical information recording/reproducing apparatus capable of effectively reducing an offset produced in a tracking error signal because of light diffracted by recording marks on adjacent tracks.
2. Description of the Related Art
An optical information recording/reproducing apparatus for recording/reproducing information by irradiating a recording medium such as a phase change medium with light beams has actually been used as an optical disk apparatus. Information is recorded on an optical disk with a recording mark formed by means of the phase change in the recording film caused by irradiation by light beams. This recorded information is reproduced by detecting a change in intensity of the reflected light beams from the optical disk depending on existence of the recording mark.
In a conventional general optical disk apparatus, recording/reproducing of information is performed only by either a groove or a land on an optical disk. The groove is for guiding light beams and formed on the optical disk along a circumferential direction thereof in a shape of concentric circles or spiral. The land is an area held between grooves having a height different from the groove. Only using either the groove or the land is done for recording/reproducing operations.
In the conventional optical disk, tracks in which recording marks are formed exist only in the groove or the land. In the optical disk on which information is recorded/reproduced only using the groove, adjacent tracks are adjacent grooves sandwiching a land, while in the optical disk having tracks in the lands adjacent lands sandwiching a groove are the adjacent tracks. In these cases, track intervals are equal to land or groove widths.
In recent years, research and development have been directed toward higher density recording in an optical disk apparatus. As a method of this, a land and groove recording system for recording information both in a land and a groove is known (Japanese Patent Application KOKAI Publication No. 57-50330). According to this land and groove recording system, there are almost no track intervals because tracks exist both in the groove and the land and track pitches are very narrow. Reproduction of recorded information is, as in the case of the conventional apparatus, performed by detecting a change in intensity of reflected light beams from an optical disk. Because of the narrow track pitches, however, cross talk due to reflected lights from recording marks on adjacent tracks is a problem. In order to deal with this problem, there has been presented a technique of reducing cross talk by properly devising a structure of the recording medium and a depth of the groove.
Reproduction of recorded information is performed by detecting a change of intensity of reflected light beams from the optical disk as in conventional ones. However, this has the problem of crosstalk caused by the reflection of light from the recording mark on an adjacent track because of a narrow track pit. For the problem of crosstalk, a crosstalk reduction technique by improving a structure of the recording medium and a depth of the groove is proposed.
Tracking control is essential to recording/reproducing of information in the optical disk. This tracking control is effected by detecting a tracking error signal indicating a shift in a position of light beams irradiated on the optical disk from a track center in a track width direction thereof from reflected light beams from the optical disk and by controlling an irradiation position of the light beams irradiated on the optical disk by using this tracking error signal to the track center. Conventionally, as a method of detecting tracking errors, a push-pull method has often been used. In the push-pull method, imbalance, in intensity of diffracted light reflected from the optical disk and caused by the groove or the like and produced due to a shift in the position of the light beams irradiated on the optical disk from the track center, is detected by obtaining a difference between two outputsignals from the 2-divided photodetector receiving the reflection light beams from the optical disk.
In the case of the land and groove recording system, however, since a considerable amount of light diffracted by the recording marks on the adjacent tracks leaks to the reflected light beams, an offset may be produced by the recording marks on the adjacent tracks if detection of tracking errors is performed by means of the push-pull method. Thus, influence exerted on the tracking error signal by the recording marks on the adjacent tracks increases by an amount equivalent to narrowing of the track pitches relative to conventional ones. Production of an offset in the tracking error signal due to the light diffracted by the recording marks on the adjacent tracks occurs when the recording marks exist only on the adjacent tracks in one side of tracks during recording/reproducing. The reason is why the light diffracted by the recording marks on only one side of the adjacent tracks enters the reflected light beams from the optical disk. Optimization of the structure of the recording medium and the depth of the groove, mentioned above, may be effective for reducing crosstalk due to the recording marks on the adjacent tracks during reproducing of information. But it will not be effective at all for reducing an offset during detecting of tracking errors.
Apparent from the foregoing, when track pitches are narrow as in the case of the land and groove recording system, an offset is produced in a tracking error signal because of light diffracted by recording marks on adjacent tracks and thus it is difficult to effect tracking control with respect to remaining errors permitted by the system.
It is an object of the invention to provide an optical information recording/reproducing apparatus capable of performing accurate tracking by reducing an offset in a tracking error signal produced due to light diffracted by recording marks on adjacent tracks even when track pitches are narrow.
According to a first aspect of the invention, an optical information recording/reproducing apparatus is provided for optically recording/reproducing information by irradiating lights on tracks on a recording medium. The apparatus is provided with detecting means for selectively detecting an area smaller than a beam cross-section of a light beam reflected by the recording medium and obtaining an output signal corresponding to the area; tracking error signal producing means for producing, from the output signal of the detecting means, a tracking error signal indicating a shift in a position of a light beam irradiated on the recording medium in a track width direction; and tracking control means for controlling an irradiation position of a light beam irradiated on the recording medium in the track width direction based on the tracking error signal. The area smaller than the reflected area is a circular area.
A preferred embodiment of the apparatus is as follows:
(1) The circular area smaller than the beam cross-section of the light beam reflected by the recording medium is a circular area having a diameter that is 0.2 to 1.0 times the beam diameter of the light beam reflected by the recording medium.
(2) When a value of a relational expression among a track pitch Tp on the recording medium, a wavelength xcex of a light beam for recording/reproducing information on the recording medium and NA of an objective lens for condensing the light beam in a very small beam spot on recording medium is Tp/xcexxc3x97NA=0.66, the detecting means selectively detects a circular area having a diameter that is 0.23 to 0.86 times a beam diameter of the light beam reflected by the recording medium and obtains an output signal corresponding to the area.
(3) When a track pitch on the recording medium is 0.75 xcexcm or 0.74 xcexcm, the detecting means selectively detects a circular area having a diameter that is 0.4 to 0.8 times a beam diameter of the reflected light beam reflected by the recording medium and obtains an output signal corresponding to the area.
(4) When a track pitch on the recording medium is 0.7 xcexcm, the detecting means selectively detects a circular area having a diameter that is 0.23 to 0.6 times a beam diameter of the light beam reflected by the recording medium and obtains an output signal corresponding to the area.
(5) The area smaller than the reflected area is an area which is symmetrical with respect to the center axis with a bisector in parallel with a track tangential direction of a photodetector as a center axis and has widths ⅓ or less of a diameter of the reflected beam. Two limited areas in the vicinity of the center of the photodetector are each used as an area having widths ⅓ or less of a diameter of the reflected beam, and the tracking error signal is produced in such a manner that with detected signals by the two areas as A and B respectively, Axe2x88x92B or (Axe2x88x92B)/(A+B) is calculated. As the two areas, two leaf-shaped areas respectively having widths ⅓ or less of a diameter of the beam spot irradiated on the photodetector in the track radial direction are used.
(6) The recording medium is structured by alternately disposing areas different in height and information is recorded in the areas as tracks.
An area, e.g., a circular area, having a diameter that is 0.2 to 1.0 times a beam diameter, of the light beam reflected by the recording medium can be a tracking error detection area, and the tracking error can be detected by receiving the reflected light beams from the area by the 2-divisional detector and using the difference of the outputs. In this case, an offset in the tracking error signal caused by the light diffracted by the recording marks on the adjacent tracks is set off when a tracking error signal is produced, making it possible to effectively reduce an offset.
Furthermore, according to the invention, for the tracking error detection area, two areas in vicinity of the center and symmetrical in the divisional line of the photodetector divide a line parallel to a track direction. This makes it possible to reduce an offset more compared with the case where a difference in output between a right and a left half of the photodetector of conventional push pull method is used for a tracking error signal and to perform stable tracking control. Therefore, since in the land and groove recording system, as an area for producing a tracking error signal on the photodetector, the areas in the vicinity of the center thereof and asymmetrical in the disk radial direction are specified, it is possible to reduce a track offset produced when a mark is recorded only in either one of the tracks adjacent to a target track, improving track follow-up performance.
As described above, according to the first aspect of the invention, tracking errors are detected by using a circular area with a diameter, for instance between 0.23 to 0.8 times a diameter of the light beams reflected by the recording medium or by setting two areas in each area of both sides as tracking area detecting means, in which the reflected light beams are divided by the line parallel to the track direction extending from the center of the reflected light beams, with a position away from the beam center of the reflected light beams by 0.18 to 0.5 times a beam diameter thereof as a center and not in contact with the center of the reflected light beams or an outer end thereof. Therefore, even when track pitches are narrow as in the case of the land and groove recording system, an offset produced in a tracking error signal because of light diffracted by the recording marks on the adjacent tracks can be effectively reduced and tracking can be accurately performed.
According to a second aspect of the invention, an optical information recording/reproducing apparatus is provided for optically recording/reproducing information by irradiating light beams on tracks on recording medium. The apparatus is provided with detecting means for selectively detecting two areas with a position away by 0.18 to 0.5 times a beam diameter in both sides of a track width direction from a beam center of a light beam reflected by the recording medium as a center and not in contact with the beam center or an outer end of the reflected light beam and obtaining an output signal corresponding to the areas; tracking error signal producing means for producing, from the output signal of this detecting means, a tracking error signal indicating a shift in a position of the light beam irradiated on the recording medium in a track width direction; and tracking control means for controlling an irradiation position of the light beam irradiated on the recording medium based on the tracking error signal.
A preferred embodiment of the apparatus is as follows:
(1) When a track pitch on the recording medium is 0.75 xcexcm, the detecting means selectively detects two areas with a position away by 0.18 to 0.5 times a beam diameter in both sides of the track width direction from a beam center of the reflected light beam reflected by the recording medium as a center and not in contact with the center or an outer end of the reflected light beam and obtains an output signal corresponding to the areas.
(2) When a track pitch on the recording medium is 0.74 xcexcm, the detecting means selectively detects two areas with a position away by 0.18 to 0.45 times a beam diameter in both sides of the track width direction from the beam center of the reflected light beam reflected by the recording medium as a center and not in contact with the center or an outer end of the reflected light beam and obtains an output signal corresponding to the areas.
(3) When a track pitch on the recording medium is 0.7 xcexcm, the detecting means selectively detects two areas with a position away by 0.18 to 0.4 times a beam diameter in both sides of the track width direction from the beam center of the reflected light beam reflected by the recording medium as a center and not in contact with the center or an outer end of the reflected light beam and obtains an output signal corresponding to the areas.
(4) The recording medium is structured by alternately disposing areas different in height and information is recorded in the areas as tracks.
As described above, the tracking error is detected by setting two areas in each area of both sides, in which the reflected light beams are 2-divided by the line parallel to the track direction extending the center of the reflected light beams, with a position away from the beam center of the reflected light beams by 0.18 to 0.5 times a beam diameter thereof as a center and not in contact with the center of the reflected light beams or an outer end thereof, as a tracking error detection areas. An offset in the tracking error signal caused by the light diffracted by the recording marks on the adjacent tracks is set off when a tracking error signal is produced, making it possible to effectively reduce an offset.
Therefore, according to the second aspect of the invention, the tracking error detection is, as in the case of the land and groove recording system, effective in particular when track pitches are very narrow. More specifically, since an offset in the tracking error signal attributed to the light diffracted by the recording marks on the adjacent tracks is effectively reduced, it is possible to perform recording/reproducing of information in narrower track pitches, consequently increasing a recording density.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.